Techniques to improve communications between a base station and a mobile station using a relay station have been studied. A relay station may be used to improve coverage and throughput in the system. The relay station can assist a base station in communicating with a mobile station that may be experiencing very poor signal conditions, leading to coverage improvements in the network. Throughput improvements may be achieved with the relay station in general by increasing the capacity of the link to the mobile station.
One particular technique to improve communications using the relay station is referred to as “cooperative relaying”, which involves simultaneous reception of signals transmitted from the base station and the relay station at the mobile station. As an example, cyclic delay diversity may be used in an orthogonal frequency division multiplexing (OFDM) system where the signal from the relay station is a cyclically delayed version of the signal from the base station. Such techniques generally depend on the signals from the relay station and the base station arriving at the mobile station at approximately at the same time with any time misalignment being small enough to be accommodated by the receiver of the mobile station.
In general, it is assumed that the transmissions from the relay station coincide with the transmissions from the base station at the mobile station, and that any disparities in arrival times due to the propagation distance differences are small enough so as to not create any particular problems. For example, in an OFDM system, it is generally assumed that the arrival time difference between the base station and relay station signals at the mobile station is smaller than the cyclic prefix portion of the transmissions.
One way to achieve this goal of having the signals arrive at the same time is to set the signal transmit time at the relay station to the time the relay station would receive the signal from the base station. In this instance, the misalignment becomes smaller as the distance between the relay station and the mobile station decreases, with a perfect alignment being achieved when the relay and the mobile stations are co-located.
A second solution is for the mobile station to go through a ranging process involving signaling and communication with the relay station so that the relay station can compute the correct transmit time for the mobile station. The relay station sends a signal to the mobile station so that the mobile station can measure the time of arrival of the signal and compare it with the time of arrival of a transmission from the base station. The mobile station then can signal the correction in transmit time required back to the relay station.
With the first solution, the assumption that the difference in arrival times is small enough so that the performance is not affected is not necessarily true in all situations. Consider the scenario shown in FIG. 1, which illustrates an example wireless network 100 which includes a base station 110 and a relay station 120 providing communication services to mobile stations 130-1 and 130-2. Also illustrated are communication links among the base station 110, the relay station 120 and the mobile stations 130. The value DBS on the link between the base station 110 and the relay station 120 represents a propagation delay of a signal originating from the base station 110 to the relay station 120. The subscripted text for each delay value refers to the link and the direction for which the delay is represented by the variable. The propagation delay of the links are assumed to include any non-line-of-sight effects. Also, since propagation delay is primarily a function of the environment, the direction of the link does not affect the propagation delay. For example, the propagation delay between the base station 110 and the first mobile station 130-1 is the same whether the signal is transmitted from the base station 110 to the mobile station 130 or vice-versa (i.e., DBM=DMB).
Typically, the relay station 120 transmits the signal for both mobile stations 130 at the same time. It is extremely unlikely that both mobile stations 130 will be at a same distance from the relay station 120. Thus, the propagation delays between the relay station 120 and the respective mobile stations 130-1 and 130-2 will not be equal, i.e., DRM≠DRM2. Thus, if the relay station 120 uses the same transmit time for both mobile stations 130, the transmission from the base station 110 and the relay station 120 will not be aligned for at least one of the mobile stations 130. This problem is compounded as the number of the mobile stations 130 increase.
For large cells, the distances between the relay station 120 and the mobile stations 130 can be in the order of 600 meters or more. This corresponds to a propagation delay of 2 microseconds. Depending on the location of the mobile station 130 in relation to the base station 110 and the relay station 120, a timing misalignment of up to ±2 microseconds can result. For a Long Term Evolution (LTE) system with a cyclic prefix period of 4.69 microseconds, this would leave significantly less room for the delay spread of the relay station-mobile station channel. If the channel has a path delay of 3 microseconds, the path will fall outside the cyclic prefix period when the mobile station 130 receives the combined signals from the relay station 120 and the base station 110. Thus, the first solution is unsatisfactory.
A shortcoming of the second solution of using the ranging process between the mobile station 130 and the relay station 120 is that the solution precludes simple system solutions where the relay station 120 has minimal impact on the frame structure and signaling in the system. Since the mobile station needs to implement new functionality and new signaling messages, it is not possible to incorporate relay stations in a legacy system in a way that works with older mobile stations. This limits the advantages of using relay stations in some cases. Even for systems, where such compatibility requirements with legacy mobile stations don't exist, such control procedures can consume valuable system resources. In other words, this solution can severely increase the complexity of the system.